The utilisation of bridged P-heterocycles in the fragmentation-related phosphorylation of nucleophiles forms a special synthetic technique (Scheme 1). The precursors of the methylenephosphine oxides (3), the phosphabicyclooctene derivatives (6-8) could be obtained by the Diels-Alder reaction of dihydrophosphinine oxides (5) and dienophiles, such as dimethyl acetylenedicarboxylate (DMAD) or maleic acid derivatives (Scheme 2). Novel cycloadducts, such as dimers (9) and diaza derivatives (10) have also been described. Due to their more strained ring, the phosphabicyclooctadienes (6) are more unstable thermally, than the bicyclooctenes (7, 8) are (Scheme 3). The phosphabicyclooctadienes (e.g. 6a) were useful in the thermoinduced fragmentation-related phosphinylation of phenole derivatives (Scheme 4). Under photochemical conditions, the phosphinylated products could be obtained already at room temperature. The use of phosphabicyclooctenes (e.g. 7a) as the precursor and simple alcohols or primary amines as the nucleophiles led to the formation of phosphinic esters 15 and amides 16, respectively, in good yields (Scheme 5). Regarding the photolyses, a novel mechanism involving an intermediate with a pentavalent, pentacoordinated phosphorus atom (19) was substantiated (Scheme 6). The phosphabicyclooctenes with trialkylphenyl substituent on the phosphorus atom (20) were of special importance in the proof of the novel addition-elimination mechanism. The reaction of the triisopropylphenyl- dihydrophospinine-oxide (21) with DMAD did not afford the [4+2] cycloadduct (22) expected, rather a phosphorane/ylide (23) was formed (Scheme 7) through spirocyclic intermediates 241 or 242. This type of inverse Wittig-reaction has never been observed earlier and proved to be general for a variety of P-heterocycles (25A-D) with trialkylphenyl substituent on the phosphorus atom (Scheme 8).
|Translated title of the contribution||[4+2] versus [2+2] cycloadditions in the sphere of P-heterocycles|
|Number of pages||3|
|Journal||Magyar Kemiai Folyoirat, Kemiai Kozlemenyek|
|Publication status||Published - Mar 2004|
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